Accompanying the requirements of film users for wrappings that combine a higher barrier effect against water vapour and oxygen permeation but with reduced use of material, also characterised by the recent appearance of tubular films with small diameters of 20 to 150 mm, is the trend towards combining material properties by means of coextruding different polymers in order to satisfy these demands. In the process, biaxially stretched polyamide/polyolefin composites with wall thicknesses between 35 and 70 .mu.m are increasingly gaining general acceptance.
The problem with polyamides in this type of film structure even during biaxial stretching using the known "double bubble" technique, is the termination of transverse expansion of the tubes, due to molecular orientation and expansion-induced crystallisation, at degrees of transverse stretching between 2.8 and 4. When the tubular films are used for the specific purpose of, for instance, wrapping sausages, then the mechanical properties of the stretched polyamide, as compared with that of the incompletely stretched polyolefin, dominate.
When modifying this type of PA/PE film composite by admixing organic or inorganic filler additives, especially to the internally or externally located PA layer(s), narrow limits are set with regard to fractional amount and granular size because a polyamide/polymer matrix of this type is weakened if fractional amounts, depending on the specific additive, are exceeded and problems then occur during the production-determining stretching process.
On the other hand, this restriction on amounts when admixing fillers in the past frequently led to wrappings whose mix components were not effective enough such that optimisation of specific film properties could not be fully achieved and detrimental other properties and/or undesirable production difficulties had to be accepted.
Thus, for example, the addition of increased proportions of pigment to an internally or externally located polyamide layer, with the objective of increasing the intensity of colour, led to a number of production engineering difficulties, which were obvious to the user due to reduced product quality and to the manufacturer due to the reduced working life of plant.
Pigment granules, which are located on the surface of the polymer tubes during the stretching process, lead to micro-tears in the surface. These micro-tears are often the reason why stretching film bubbles burst, because they are the starting points for uncontrollable tear propagation.
Deposits of pigment granules are produced on the lips of extrusion dies, especially with high pigment contents in one of the internally or externally located polymer layers. These deposits again produce irregularities in the thickness over the circumference of the primary tube being stretched and thus produce on the one hand increased striations in the film end product and on the other hand production engineering difficulties associated with the biaxial stretching process such that the thinner areas have a different deformation characteristic and this often causes the stretching film bubbles to burst.
Increased pigment granule contents in externally located polymer layers are especially detrimental if additive deposits from the pigment are formed on the transport and squeezing rolls, causing soiling of the plant which is unacceptable in production of food-grade film.
If the highly pigmented layer is located on the internal face of the film tube, there is also the risk that pigments are transferred to the foodstuffs on direct contact of the film with the contents.
The object of the present invention was therefore to enable the production of a wrapping with an increased proportion of filler components in a polyamide (PA) layer, without having to accept disadvantages with respect to the production process, especially biaxial stretching, or the resulting qualitative characteristics of the film.